This invention relates generally to the field of signal processing, and more particularly, to a method and apparatus for filtering a signal using a lattice structure.
Lattice and ladder implementations of IIR and FIR filters have been in use for many years, for example, in speech synthesis/coding applications or to simply implement equalizing filters. The stability of these filters is preserved by the linear interpolation of their coefficients, which makes them quite attractive for filters whose coefficients must vary from frame to frame, such as in speech coding.
However, lattice filters are not immune from scaling problems which can occur, for example, when poles are close to the unit circle. In this case, signals are subjected to a high level of amplification, which can result in saturation or clipping. This is especially bothersome when implementing equalizing or shelving filters, where high bandwidths are often desirable. Many strategies have been tried to avoid the scaling problems that arise when using lattice filters. Unfortunately, these strategies have resulted in the use of additional multipliers, which increase the processing cost of the filters.
Thus, it would be desirable to have a lattice structure which could be used in IIR and FIR applications to overcome problems associated with scaling, while requiring no increases in processing costs.
The present invention provides a method and apparatus which include a xe2x80x9creverse latticexe2x80x9d structure for implementing IIR and FIR filters. The reverse lattice structure implementation of an IIR filter exhibits desirable scaling properties at no additional computational or memory cost, by scaling down signals at filter sections corresponding to reflection coefficients close to 1 in magnitude. This feature provides automatic scaling for poles lying close to the unit circle which would otherwise produce large amplification factors.
In one embodiment a reverse lattice structure is provided for processing an input signal to form an output signal. The reverse lattice structure comprises: a means to scale the input signal to form a scaled input signal; a means to delay a first feedback signal and to form a delayed feedback signal; a means to combine the delayed feedback signal with the scaled input signal to form a second feedback signal; a means to scale the second feedback signal to form a third feedback signal; and a means to combine the input signal and the third feedback signal to form the output signal.
In another embodiment, a method is provided for processing an input signal to form an output signal. The method begins by receiving the input signal and scaling the input signal to form a scaled input signal. Next, a feedback signal is received. The feedback signal is delayed to form a delayed feedback signal. The scaled input signal and the delayed feedback signal are combined to form an adjusted feedback signal, which is scaled to form a scaled adjusted feedback signal. Finally, the input signal and the scaled adjusted feedback signal are combined to form the output signal.
Other features and advantages of the present invention will be understood upon reading and understanding the detailed description of the preferred exemplary embodiments, found hereinbelow, in conjunction with reference to the drawings, in which like numerals represent like elements.